1. Field of the Invention
This invention relates to current limiting devices for general circuit protection including electrical distribution and motor control applications. In particular, the invention relates to current limiting devices that are capable of limiting the current in a circuit when a high current event or high current condition occurs.
2. Description of Related Art
There are numerous devices that are capable of limiting the current in a circuit when a high current condition occurs. One known limiting device includes a filled polymer material that exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. U. S. Pat. Nos. 5,382,938, 5,313,184, and European Published Patent Application No. 0,640,995 A1 each describes electrical devices relying on PTC behavior. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a transformation from a basically conductive material to a basically resistive material. In some of these prior current limiting devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes.
U.S. Pat. No. 5,614,881, to Duggal et al., issued Mar. 25, 1997, the entire contents of which are herein incorporated by reference, discloses a current limiting device. This current limiting device relies on a composite material and an inhomogeneous distribution of resistance structure.
Current limiting devices are used in many applications to protect sensitive components in an electrical circuit from high fault currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response time, alternately known as switching time, to minimize the peak fault current that develops.
In operation, current limiting devices are placed in a circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a high current condition occurs, the PTCR material heats up through resistive heating until the temperature is above the xe2x80x9cswitch temperature.xe2x80x9d At this point, the PTCR material resistance changes to a high resistance state and the high current condition current is limited. When the high current condition is cleared, the current limiting device cools down over a time period, which may be a long time period, to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future high current condition events.
Known current limiting devices comprise electrodes, electrically conductive composite material, a low pyrolysis or vaporization temperature polymeric binder and an electrically conducting filler, combined with an inhomogeneous distribution of resistance structure. The switching action of these current limiting devices occurs when joule heating of the electrically conducting filler in the relatively higher resistance part of the composite material causes sufficient heating to cause pyrolysis or vaporization of the binder.
During operation of known current limiting devices, at least one of material ablation and arcing occur at localized switching regions in the inhomogeneous distribution of resistance structure. The ablation and arcing can lead to at least one of high mechanical and thermal stresses on the conductive composite material. These high mechanical and thermal stresses often lead to the mechanical failure of the composite material.
Further, electrically conductive composite materials that have been used in known current limiting devices are often quite brittle, and may fracture during high voltage and high current events. Also, there is often little reproducibility in electrically conductive composite material batches, which have been previously used in current limiting devices. Accordingly, the characteristics of electrically conductive composites in current limiting devices vary, and may adversely effect the operation and reliability of operation of the current limiting device.
One such composite material, previously attempted for use in current limiting devices is Epotek N30 (Epoxy Technologies Inc.), a commercially available epoxy. Epotek N30 is filled with nickel particles to provide electrical conductivity. Several batches Epotek were tested, and some of the batches were found to give good electrical performance. However, there was little or no reproducibility from batch to batch of Epotek N30. Further, the Epotek N30 batches were quite brittle, thus resulting in fracture during testing.
Therefore, electrically conductive composite materials for use in current limiting devices should possess desirable, constant and reproducible electrical and mechanical properties, which are suitable for high current multiple use current polymer limiting devices. These electrical and mechanical properties include, but are not limited to desirable current limiting device properties, such as a low initial contact resistance, high switch resistance, switching times that are less than a few milli-seconds, and also mechanical toughness and durability.
Accordingly, it is desirable to provide a quick, reusable current limiting device, where the current limiting device overcomes the above noted, and other, disadvantages of the related art.
It is further desirable to provide a current limiting device, where the composite material possesses desirable electrical and mechanical properties suitable for a multiple use current polymer limiting device. These electrical and mechanical properties include, but are not limited to, low initial contact resistance, high switch resistance, switching times that are less than a few milli-seconds and mechanical toughness and durability so that the polymer current limiting device has multiple use capability.
It is also desirable to provide a high current multiple use current limiting device. The device comprises at least two electrodes; an electrically conducting composite material between said electrodes; interfaces between said electrodes and said composite material; an inhomogeneous distribution of resistance at said interfaces whereby, during a high current event, adiabatic resistive heating at said interfaces causes rapid thermal expansion and vaporization of the binder resulting in at least a partial physical separation at said interfaces; and means for exerting compressive pressure on said composite material. The electrically conductive composite material comprises an organic binder portion having a high Tg epoxy and a low viscosity polyglycol epoxy; at least one epoxy curing agent; and a conductive powder.
Further, it is desirable to provide an electrically conductive composite and method of manufacture of the electrically conductive composite, where the electrically conductive composite material comprises an organic binder portion having a high Tg epoxy and a low viscosity polyglycol epoxy; at least one epoxy curing agent; and a conductive powder
These and other advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, disclose preferred embodiments of the invention.